Multi-stage raman amplifier

ABSTRACT

A Raman amplifier includes at least a first and a second optical Raman-active fiber disposed in series with each other. A first pump source is connected to the first Raman-active fiber, and is adapted for emitting and coupling into the first Raman-active fiber a first pump radiation including a first group of frequencies. A second pump source is connected to the second Raman-active fiber, and is adapted for emitting and coupling into the second Raman-active fiber a second pump radiation including a second group of frequencies. The whole of said first and second group of frequencies extends over a pump frequency range having a width of at least the 40% of the Raman shift. The minimum and the maximum frequency in each of said first and second group of frequencies differ with each other of at most the 70% of said Raman shift.

The present invention relates to a method for amplifying optical signalsin a multi-stage Raman amplifier and to a multi-stage Raman amplifier.In particular, the present invention relates to a method for amplifyingoptical signals in a lumped multi-stage Raman amplifier and to a lumpedmulti-stage Raman amplifier.

The maximum number of dense wavelength-division-multiplexed (DWDM)signals that can be transmitted over a single optical fiber has beenrapidly increasing over the last few years. This trend, coupled with anincreasing data rate per signal, has lead to a profound increase in theamount of signal power propagating through such optical fibers, in orderto sustain applications such as data communications and the Internet.This has created a simultaneous demand for large bandwidth and highoutput power from the optical amplifiers used in such systems.

Erbium-doped fiber amplifiers (EDFAs) are a relatively maturetechnology. The amount of bandwidth that such amplifiers can produce,however, is fundamentally limited by the physics of the erbium atomsthat produce the optical gain in such devices.

Raman amplifiers offer an alternative to EDFAs and are recentlyattracting much attention in DWDM systems, due to their distinctiveflexibility in bandwidth design and growing maturity of high-power pumpmodule technology. Raman amplifiers offer several advantages: low noise,flexible use of signal wavelengths (since the Raman gain peak is mainlydependent on the pump wavelength and not on the emission cross sectionof a dopant) and a broad gain bandwidth (multiple pumps can beemployed). In particular, multi-wavelength pumping allows to extend thewavelength range over which flat Raman gain can be achieved: the totalgain profile of such amplifiers consists of a superposition of thecontributions from each individual pump.

On the other hand, many factors must be considered in the design of theamplifier and systems that use them. A thorough understanding of somekey factors is required, such as, for example, pump-to-pump powertransfer, signal-to-signal power transfer, pump depletion, doubleRayleigh scattering (DRS) and amplifier spontaneous noise. H. Kidorf etal., in their article “Pump interactions in a 100-nm Bandwidth Ramanamplifier”, IEEE Photonics Technology Letters, vol. 11, no. 5, pag.530-2 (1999), disclose a computer model that simulates all the physicalproperties that affect the above listed factors. The computer modelnumerically solves a differential equation. The authors used their modelin order to design a distributed Raman amplifier with a 100-nm bandwidthand with minimum gain variation. The amplifier was designed to have atotal output power of 50 mW for 100 channels spaced 1 nm/channel. Theintended use of the amplifier was the compensation of 45-km fiber spans(intended for 10.000 km transmission) made of pure silica core fiberplus an extra 3 dB to compensate for internal losses (WDM coupler,isolator, etc.). In a first attempt, the authors tried to evenly spaceeight pumps between 1432 and 1516 nm. With the goal of implementing theamplifier with semiconductor pumps, a maximum pump power of 120-130 mWper pump was chosen. According to the authors, the result of thissimplistic design was a very poor amplifier: the gain variation of theamplifier was 10.5 dB due to power being transferred from the lowwavelength pumps to the high wavelengths pumps. The added power in thehigh wavelengths pumps caused excessive gain at the higher signalwavelengths. Through iterative modeling, the authors arrived at a pumpscheme whereby a large energy density at low wavelengths provides pumppower for both the high wavelength pumps and the low wavelength signals.By properly balancing the pump's spectral density, an amplifier with apeak-to-peak gain ripple of 1.1 dB was designed.

Another known approach for obtaining a flat Raman gain on broadbandwidths using multi-wavelength pumping is to carefully select themagnitude of each contribution in order to achieve the desired gainprofile. For example, P. M. Krummrich et al, “Bandwidth limitations ofbroadband distributed Raman fiber amplifiers for WDM systems”, OFC 2001,vol. 1 pag. MI3/1-3, analyze the impact of pump interactions bothnumerically and experimentally. Their numerical model works byintegrating a set of coupled differential equations describing thepropagation of pump and signal radiation in the transmission fiber. Forthe experiments, the authors use a multi-channel pump unit. The pumpradiation in the wavelength range of 1409-1513 nm is generated by highpower laser diodes and combined by a WDM coupler. More particularly,seven pump channels have been used in a counter-directional pumpconfiguration to achieve flat gain in the wavelength range of 1530-1605nm with the following set of pump wavelengths: 1424, 1438, 1453, 1467,1483, 1497 and 1513 nm. The launch powers have been adjusted to achievean average gain of 10 dB with a gain variation of less than 0.5 dB.According to the authors, the strongest impact of pump interactions canbe observed for the channels with the shortest and longest wavelength.The pump channel at 1424 nm experiences 11 dB of additional loss and thechannel at 1513 nm experiences 7 dB gain. Further, according to theauthors, should the overall gain be increased, it is quite difficult topredict which pump diode output power has to be increased by whichamount, due to the energy transfer between the pumps. For gain valueshigher than 10 dB, due to the strong impact of pump interactions and theresulting gain tilt, the gain at the long wavelength side always growsstronger than the gain at the short wavelength side if any of the pumplaser output powers is increased. According to the authors, this effectlimits the maximum value of flat gain for a signal wavelength range of75 nm to approximately 19 dB.

Another numerical model is disclosed in X. Zhou et al., “A SimplifiedModel and Optimal Design of a Multiwavelength Backward-Pumped FiberRaman Amplifier”, IEEE Photonics Technology Letters, vol. 13, no. 9,pag. 945-7 (2001). The authors obtain a closed-form analyticalexpression for pump power evolution. Based on the obtained analyticalexpression, formulas for calculating the small-signal optical gain andnoise figure are then presented. The application of the developed modelin pump optimization design is also discussed. In order to obtain thedesign for a wide-band optical amplifier having a flat gain, the authorsuse the following parameters: maximal pump light frequency 214.2 THz(1400 nm), minimal pump light frequency 200 THz (1500 nm), fiber loss atthe pump frequency 0.3 dB/km, fiber loss at the signal frequency 0.2dB/km, fiber length 10 km, required gain 20 dB, number of channels 100(from 1510 to 1610 with 1 nm separation), fiber effective area 50 μm².By considering three pump wavelengths, the authors obtain the optimalpump wavelength at 1423, 1454 and 1484 nm, and the corresponding optimalpump power as 1.35, 0.19 and 0.20 W, respectively. By considering sixpump wavelengths, the authors obtain the optimal pump wavelength as1404, 1413, 1432, 1449, 1463 and 1495 nm, and the corresponding optimalpump power as 0.68, 0.6, 0.44, 0.19, 0.076 and 0.054 W, respectively. Itis shown that the gain ripple can be compressed definitely by increasingthe number of pump light sources. However, the noise performance of thesix-pump amplifier is worse than that of the three-pump amplifier.According to the authors, this is because the six-pump amplifier has apump at higher wavelength (1495 nm). More particularly, FIG. 1 of thearticle shows a noise figure ranging from about 7.5 dB at 1510 nm toabout 4 dB at 1610 nm for the six-pump amplifier. On the other hand,FIG. 1 of the article shows a gain variation of about 5 dB in the wholewavelength range for the three-pump amplifier.

US patent application no. 2002/0044335 discloses an amplifier apparatusincluding an optical transmission line with a Raman amplification regionthat provides a pump to signal power conversion efficiency of at least20%. The Raman amplification region is configured to amplify a signalwith multiple wavelengths over at least a 30 nm range of wavelengths,preferably over at least a 50 nm range of wavelengths, more preferablyover at least a 70 nm range of wavelengths. A pump source is coupled tothe optical transmission line. An input optical signal is amplified inthe Raman amplification region and an output signal is generated thathas at least 100 mW more power than the input optical signal. In onedisclosed embodiment, the amplifier apparatus had more than 3.2 dB ofgain over 105 nm utilizing a Lucent DK-20 dispersion compensating fiber.1 dB of loss was assumed to be present at both ends of the gain fiber.This fiber was pumped with 250 mW at 1396, 1416 and 1427 nm, 150 mW at1450 nm, 95 mW at 1472 nm and 75 mW at 1505 nm. Ten input signals at1520, 1530, 1540, 1550, 1560, 1570, 1580, 1590, 1600 and 1610 nm had 12mW of power and were counter-propagating with respect to the pumpwavelengths. FIG. 6 of the '44335 patent application shows the noisefigure of this amplifier, ranging from about 7.5 dB at 1520 nm to about6 dB at 1610 nm. In an embodiment disclosed as wall band Ramanamplifier, the amplifier apparatus includes a transmission line with twoRaman amplification regions. Two WDMs are provided. Shorter signalwavelengths can receive more gain in one of Raman amplification regionsthan in the other. A lossy member can be positioned between the Ramanamplification regions. The lossy member can include at least one of anadd/drop multiplexer, a gain equalization element, an optical isolatoror a dispersion compensating element. One WDM receives a first set ofpump wavelengths provided by a first pump source and the WDM between thetwo amplification regions receives a second set of pump wavelengthswhich can provide gain to the optical signal and extract optical energyfrom at least a portion of the first set of pump wavelengths. The secondset of wavelengths is provided by a second pump source. The WDM betweenthe two amplification regions can substantially pass signal wavelengthsas well as at least a portion of the first set of pump wavelengthsbetween the two Raman amplification regions. According to the authors, again flatness of the all band Raman amplifier can be optimized by a gainflattening filter, and/or by pump wavelengths, pump powers, the numberof pumps and the lengths of Raman gain fibers.

JP patent application no. 2001-109026 discloses a fiber Raman lightamplifier using tellurite glass as a light amplification medium. Moreparticularly, it discloses a fiber Raman amplifier constituted by usingthree lines of tellurite fibers whose lengths are respectively 150 m andthree exciting light sources whose wavelengths are respectively 1370 nm,1400 nm and 1430 nm, and by connecting three units in which thetellurite glass fibers and the exciting light sources are respectivelycombined in series and the amplifier is excited by making respectiveinput power of exciting lights to be 200 mW (totally 600 mW). A gainequal to or larger than 23 dB is obtained in a 100 nm band extendingfrom 1.5 to 1.6 μm.

A further problem related to Raman amplifiers is that backward andmulti-wavelength pumping scheme results in larger noise figures forshorter wavelength signals, due to pump-to-pump stimulated Ramanscattering, in addition to thermal noise and wavelength-dependence offiber attenuation coefficients. C. R. S. Fludger et al., “FundamentalNoise Limits in Broadband Raman Amplifiers”, OFC 2001, MA5/1-3, showthat broadband discrete Raman amplifiers based on silica-germania willhave a noise figure significantly greater than the quantum limit.According to the authors, in a discrete Raman amplifier the wavelengthdependence of the noise figure is determined by four main factors. Theseinclude the gain spectrum and component losses at the amplifier input.Also, stimulated Raman scattering (SRS) will transfer power from theshorter wavelength pumps to the longer wavelength pumps. Finally, noisefigure is also affected by increased spontaneous emission due to thethermal distribution of phonons in the ground state. In particular, if apump provides a large amount of gain to a closely spaced signal therewill be a large increase in the excess spontaneous noise. If the ratioof the gain given by that pump to the overall total gain from all thepumps is small, the noise figure will tend to 3 dB. The authors evaluatethe best achievable internal noise figure for a five wavelength-pumpeddiscrete Raman amplifier. The relative gains from each pump were chosento give the broadest and flattest spectrum with the highest pumpwavelength at 1495 nm and the lowest signal near 1500 nm. It is shownthat since a substantial amount of the total gain at shorter signalwavelengths is given by the 1495 nm pump, there is increased spontaneousemission as the signals approach the pump. At room temperature, theinternal noise figure of the amplifier is between 5 and 6 dB below 1520.However, the total noise figure of the amplifier will be greater thanthis once fiber loss and the insertion loss of components at theamplifier input are included.

S. Kado et al., “Broadband flat-noise Raman amplifier using low-noisebi-directionally pumping sources”, ECOC 2001, propose and experimentallydemonstrate an optimized bi-directional pumping scheme that realizes aless than 0.7 dB flatness over C- and L-bands of both Raman gain andoptical noise figure, simultaneously. In order to use forward-pumping,for the proposed method, a new type of pump laser having low relativeintensity noise (RIN) is also developed. Such laser is awavelength-stabilized multimode pump laser, where the laser chip has aninternal grating layer along laser cavity for selecting more than threelongitudinal modes. The RIN of the developed laser is more than 20 dBlower than a usual fiber Bragg grating stabilized laser. According tothe authors, this development allows to use forward pumping schemewithout significantly hurting the signal quality due to poor RINcharacteristics.

Known configurations of Raman amplifiers, such as those presented above,may achieve high and flat gain in broad wavelength ranges. However,typically this goes with an unbalanced noise figure, having highervalues for shorter signal wavelengths and lower values for longer signalwavelengths (except for the amplifier disclosed by Kado et al., that usea special type of pump laser in order to obtain a flat noise figure).The Applicant notices that the higher noise figure of shorter signalwavelengths may raise problems in some configurations of optical systemincluding Raman amplifiers, in that at least a portion of the shortersignal wavelengths may risk going outside system specifications.

Further, known configurations of Raman amplifiers, especially thoseachieving a flat gain on a broad band of wavelengths, typically use veryhigh power values for shorter pump wavelengths (in excess of 500 mW),due to transfer of energy between shorter pump wavelengths and longerpump wavelengths. The Applicant notices that this is not an optimalsolution. In fact, reliable semiconductors lasers having a poweremission in excess of 500 mW are now hardly available on the marketand/or costly, so that multiple lasers having lower power emissionshould be used. As a consequence, the cost of the overall amplifierand/or the space occupied by the pump sources may disadvantageouslyincrease.

The Applicant has tackled the problem of lowering the noise figure ofshorter wavelength signals, at the same time maintaining a low value ofnoise figure of longer wavelength signals, in a multi-wavelength pumped,high gain, broadband Raman amplifier, without the necessity of using aspecial type of laser. The Applicant has also tackled the problem ofreaching such low noise figure and high gain with relatively low powerper pump wavelength.

The Applicant has found that it is possible to lower the noise figure ofshorter signal wavelengths by splitting the optical path in which theRaman gain is obtained in at least two optical path portions. Thedifferent optical path portions are pumped by respective pumpingradiations having optical frequencies included in respective frequencyranges. Each frequency range does not extend more than 70% of the Ramanshift of the Raman gain material. The transfer of energy between shorterand longer wavelength pumping radiations due to stimulated Ramanscattering is, in such manner, greatly reduced in each optical pathportion. The lowering of energy transfer between shorter and longerwavelength pumping radiations makes practically unnecessary to launchthe shorter wavelength pumping radiations with a higher energy. This hasthe effect of lowering the noise figure of the shorter signalwavelengths, with the further advantage that a lower power per pumpwavelength can be used also for the shorter wavelengths pumpingradiations. Since the shorter wavelength pumping radiations do not workin saturation regime in any of the optical path portions wherein Ramangain is obtained, a lower noise figure of the amplifier is obtained.Furthermore, a broad and flat Raman gain may be obtained by arrangingthe pump sources so as to provide substantially the same pumping powerper wavelength. This is of particular advantage, in that all the neededpump sources may have the same average reliability and lifetime.

In a first aspect, the invention relates to a method for amplifying anoptical signal having frequency in a signal frequency range, said methodcomprising

-   -   introducing said optical signal respectively into at least a        first and a second optical paths disposed in series with each        other, each comprising a Raman-active material having a        predetermined Raman shift;    -   introducing into said first optical path a first pump portion,        said first pump portion including a first group of pump        frequencies between a first minimum pump frequency and a first        maximum pump frequency;    -   introducing into said second optical path a second pump portion,        said second pump portion including a second group of pump        frequencies between a second minimum pump frequency and a second        maximum pump frequency, a whole of said first and second group        of frequencies extending over a pump frequency range having a        width of at least the 40% of said Raman shift;        the method being characterized in that    -   at least a portion of said first group of frequencies is not        included in said second group of frequencies and at least a        portion of said second group of frequencies is not included in        said first group of frequencies;    -   said steps of introducing said first and second pump portions        into said first and second optical paths are performed such that        a residual of said second pump portion entering into said first        optical path has a power lower by 10 dB than said first pump        portion, and such that a residual of said first pump portion        entering into said second optical path has a power lower by 10        dB than said second pump portion;    -   said first minimum pump frequency and said first maximum pump        frequency differ with each other of at most the 70% of said        Raman shift; and    -   said second minimum pump frequency and said second maximum pump        frequency differ with each other of at most the 70% of said        Raman shift.

Preferably, said steps of introducing said first and second pumpportions into said first and second optical paths are performed suchthat a residual of said second pump portion entering into said firstoptical path has a power lower by 13 dB than said first pump portion,and such that a residual of said first pump portion entering into saidsecond optical path has a power lower by 13 dB than said second pumpportion.

In preferred embodiments, said first minimum pump frequency and saidfirst maximum pump frequency differ with each other of at most the 50%of said Raman shift, and said second minimum pump frequency and saidsecond maximum pump frequency differ with each other of at most the 50%of said Raman shift.

Advantageously said pump frequency range may have a width of at leastthe 50% of said Raman shift.

Preferably, said first and second group of frequencies do not overlapwith each other.

In preferred embodiments, at least one of the ranges defined between,respectively, said first minimum and said first maximum pumpfrequencies, and said second minimum and said second maximum pumpfrequencies, has a width of at least the 20% of said Raman shift.

Advantageously, the first group of frequencies is adapted for Ramanamplifying a first portion of said optical signal having a greaterattenuation versus wavelength in said Raman-active material than saidsecond portion of optical signal, amplified by said second group offrequencies.

The first and said second pump portions may be provided by a pluralityof pump lasers, said plurality of pump lasers having an overallvariation of pump power emission of at most the 50% of an average pumppower emission.

In a second aspect, the invention relates to a Raman amplifier, adaptedfor amplifying an optical signal having frequency in a signal frequencyrange, comprising at least a first and a second optical paths disposedin series with each other, each comprising a Raman-active materialhaving a predetermined Raman shift, said amplifier comprising:

-   -   a first pump source connected to said first optical path, said        first pump source being adapted for emitting and coupling into        said first optical path a first pump radiation including a first        group of pump frequencies between a first minimum pump frequency        and a first maximum pump frequency;    -   a second pump source connected to said second optical path, said        second pump source being adapted for emitting and coupling into        said second optical path a second pump radiation including a        second group of pump frequencies between a second minimum pump        frequency and a second maximum pump frequency, a whole of said        first and second group of frequencies extending over a pump        frequency range having a width of at least the 40% of said Raman        shift;    -   characterized in that    -   at least a portion of said first group of frequencies is not        included in said second group of frequencies and at least a        portion of said second group of frequencies is not included in        said first group of frequencies;    -   the couplings between said first and second pump sources and        said first and second optical paths are such that a residual of        said second pump radiation coupled into said first optical path        has a power lower by 10 dB than said first pump radiation, and        such that a residual of said first pump radiation coupled into        said second optical path has a power lower by 10 dB than said        second pump radiation;    -   said first minimum pump frequency and said first maximum pump        frequency differ with each other of at most the 70% of said        Raman shift; and    -   said second minimum pump frequency and said second maximum pump        frequency differ with each other of at most the 70% of said        Raman shift.

Preferably, the couplings between said first and second pump sources andsaid first and second optical paths are such that a residual of saidsecond pump radiation coupled into said first optical path has a powerlower by 13 dB than said first pump radiation, and such that a residualof said first pump radiation coupled into said second optical path has apower lower by 13 dB than said second pump radiation.

In preferred embodiments, said first minimum pump frequency and saidfirst maximum pump frequency differ with each other of at most the 50%of said Raman shift, and said second minimum pump frequency and saidsecond maximum pump frequency differ with each other of at most the 50%of said Raman shift.

Advantageously, said pump frequency range may have a width of at leastthe 50% of said Raman shift.

Preferably, said first and second group of frequencies do not overlapwith each other.

In preferred embodiments, at least one of the ranges defined between,respectively, said first minimum and said first maximum pumpfrequencies, and said second minimum and said second maximum pumpfrequencies, has a width of at least the 20% of said Raman shift.

Advantageously, the first group of frequencies is adapted for Ramanamplifying a first portion of said optical signal having a greaterattenuation versus wavelength in said Raman-active material than saidsecond portion of optical signal, amplified by said second group offrequencies.

In preferred embodiments, said first and said second pump sourcescomprise a plurality of pump lasers, said plurality of pump lasershaving an overall variation of pump power emission of at most the 50% ofan average pump power emission.

In a third aspect, the invention relates to an optical system comprisingat least one optical line, said optical line including at least oneoptical fiber and at least one Raman amplifier according to the secondaspect, connected to said optical fiber.

The optical system typically further comprises a transmitting stationincluding a plurality of transmitters adapted for emitting a respectiveplurality of optical channels, each having a respective wavelength, saidtransmitting station being connected to a first end of said opticalline.

The optical system typically further comprises a receiving stationincluding a plurality of receivers adapted to discriminate aninformation carried by said optical channels, said receiving stationbeing connected to a second end of said optical line.

Further features and advantages of the present invention will be betterillustrated by the following detailed description, herein given withreference to the enclosed drawings, in which:

FIG. 1 schematically shows a first embodiment of a Raman amplifieraccording to the present invention;

FIG. 2 schematically shows a second embodiment of a Raman amplifieraccording to the present invention;

FIG. 3 shows an embodiment of a Raman amplifier according to the priorart, in which the same pump radiations are used in the first and in thesecond stage;

FIG. 4 shows the noise figure obtained with two configurations of Ramanamplifier according to FIG. 3;

FIG. 5 shows the noise figure obtained with two configurations of Ramanamplifier according to FIG. 2;

FIG. 6 shows the noise figure obtained with two further configurationsof Raman amplifier according to FIG. 2;

FIG. 7 shows an experimental apparatus for measuring the Raman shift.

FIG. 1 shows a first embodiment of a Raman amplifier 10 according to thepresent invention. The amplifier 10 may be a lumped Raman amplifier.Amplifier 10 include at least two stages 10′ and 10″, disposed in serieswith each other. The following detailed description will be made withspecific reference to a Raman amplifier having only two stages: however,the skilled in the art may adapt the teachings of the present inventionto any number of needed amplifier stages, according to the differentspecifications. Odd reference numbers are used in FIG. 1 for indicatingcomponents included in the first stage 10′, whereas even referencenumbers are used in FIG. 1 for referring to components included in thesecond stage 10″. More particularly, first stage 10′ comprises a firstlength of Raman active optical fiber 11, a first pump source 13 and afirst coupling device 15.

Second stage 10″ comprises a second length of Raman active optical fiber12, a second pump source 14 and a second coupling device 16. Preferably,the first stage 10′ includes also a unidirectional device 17 adapted forpassing optical signal radiations in one direction and for blocking thesame in the opposite direction, such as an isolator or a circulator.Unidirectional device 17 may be disposed at the input and/or at theoutput of the first stage 10′. Preferably, also the second stage 10″includes a unidirectional device 18 adapted for passing optical signalradiations in one direction and for blocking the same in the oppositedirection, such as an isolator or a circulator. Unidirectional device 18may be disposed at the input and/or at the output of the second stage10″. In a preferred embodiment, a single unidirectional device is usedin the amplifier 10 between the first stage 10′ and the second stage10″. In such embodiment, the presence of the unidirectional devicebetween the first and second stage may greatly reduce the occurrence ofdouble Rayleigh scattering in the Raman amplifier 10.

The Raman amplifier 10 is suitable for amplifying a broadband WDMoptical signal, i.e. an optical signal with multiple wavelengthsincluded in a range of at least 50 nm, preferably of at least 70 nm. Awavelength range around 1550 nm may be used for the optical signal.Preferably, the optical signal may comprise one or more signalwavelengths λ_(s) greater than or equal to about 1460 nm, morepreferably greater than or equal to about 1510 nm. Preferably, theoptical signal may comprise one or more signal wavelengths λ_(s) lowerthan or equal to about 1650 nm, more preferably lower than or equal toabout 1625 nm. Alternatively, a wavelength range around 1300 nm may beused for the optical signal, for example between 1280 nm and 1340 nm.Over the wavelength range of the optical signal to be amplified, theRaman amplifier may preferably provide a gain of at least 5 dB, morepreferably of at least 10 dB, even more preferably of at least 20 dB.These preferred values of gain also apply for distributed amplification,provided that the on-off gain is considered. Over the wavelength rangeof the optical signal to be amplified, the amplifier 10 may preferablyobtain a maximum gain variation of 3 dB, more preferably of 2 dB, evenmore preferably 1 dB. Preferably, the noise figure of the amplifier 10may be lower than or equal to 8 dB, more preferably lower than or equalto 7 dB, over the wavelength range of the signal to be amplified. Forthe purposes of the present invention, by “noise figure of theamplifier” NF it has to be intended a value calculated with thefollowing formula:

${NF} = {{{P({ASE})}G\mspace{11mu} h\; v\mspace{11mu} B_{0}} + \frac{1}{G}}$

wherein P(ASE) is the power of the amplifier spontaneous emission, G isthe amplifier gain, B₀ is the receiver optical bandwidth (i.e. typicallythe bandwidth of a demultiplexer connected to a wide bandwidthphotodiode), h is the Planck's constant and ν is the signal opticalfrequency.

The Raman-active optical fibers 11, 12 are optical fibers suitable forobtaining a gain by stimulated Raman scattering. In the present detaileddescription, specific reference is made to Raman-active optical fibersfor obtaining Raman gain in the Raman amplifier 10: however, the skilledin the art may adapt the teachings of the present invention with othertypes of optical paths including any Raman-active material (i.e. amaterial being capable of providing an optical gain by stimulated Ramanscattering), such as, for example, integrated optical waveguides. Forexample, the Raman-active fibers 11, 12 may be silica-based fibers.Typically, such silica-based fibers have a core comprising germanium oranother dopant suitable for enhancing the Raman effect inside the core,such as for example phosphorus or boron. In another embodiment, theRaman-active fibers may be tellurite-based fibers. In preferredembodiments, the Raman-active fibers 11, 12 may be microstructuredfibers, i.e. fiber structures that incorporate numerous air holessurrounding a solid core. Preferably, the sum of the lengths of theRaman-active fibers 11, 12 may be lower than or equal to about 10 km,more preferably lower than or equal to about 8 km. A lower total lengthof the Raman-active fibers 11, 12 contributes to keeping the noisefigure of the amplifier 10 lower. On the other hand, in order to obtaina sufficient Raman gain, the length of each Raman-active fiber 11, 12may be at least 100 m, preferably at least 200 m, more preferably atleast 1000 m. Preferably, the effective area of the Raman-active fibers11, 12 may be lower than or equal to 60 μm², more preferably lower thanor equal to 35 μm², even more preferably lower than or equal to 20 μm².A lower effective area increases the nonlinear effects inside the coreof the Raman-active fibers 11, 12, such as the stimulated Ramanscattering, exploited for Raman amplification. Preferably, the ratiobetween the Raman gain coefficient and the effective area g_(R)/A_(eff)of the Raman-active fibers 11, 12 is of at least 1.0 1/(W·km), morepreferably of at least 3.0 1/(W·km), even more preferably of at least6.0 1/(W·m). A higher ratio between the Raman gain coefficient and theeffective area of the Raman-active fibers 11, 12 allows to obtain ahigher Raman gain per unit length of fiber. This keeps the noise figureof the amplifier tower, as a lower length of Raman-active fiber may beused for obtaining the desired signal amplification. Another importantparameter that may have an impact on the Raman amplification and, moreparticularly, on the noise figure, is the attenuation of theRaman-active fibers 11, 12: preferably, such attenuation may be lowerthan or equal to 2.0 dB/km, more preferably lower than or equal to 1.5dB/km, at the pump wavelength. At the signal wavelength, the attenuationof the Raman-active fibers may be preferably lower than or equal to 1.0dB/km, more preferably lower than or equal to 0.7 dB/km.

The pump sources 13, 14 may each comprise one or more laser diodes.Preferably, the laser diodes have high output power, i.e. at least 50mW. Preferably, the wavelength emission of the laser diodes may becontrolled, for example with an external fiber Bragg grating. Typically,the laser diodes may emit polarized pump radiation and include apolarization maintaining fiber length. In a preferred embodiment, twolaser diodes may be combined for each pump wavelength, for example by apolarization beam combiner, in order to achieve a high output power anda polarization-independent Raman gain. Several pump wavelengths may becombined together, typically by using wavelength division multiplexers(WDMs). Preferably, each pump source 13, 14 may provide an overall pumppower of at least 100 mW, more preferably of at least 200 mW. Thewavelength of the radiation emitted by the laser diode or diodesincluded in the pump sources 13, 14 is related to the signal radiationwavelengths: in order to have Raman amplification, the wavelengthemission of the laser diodes should be shifted with respect to thesignal radiation wavelengths in a lower wavelength region of thespectrum, according to the Raman shift of the material included in thecore of the Raman-active fibers 11, 12, i.e., according to the shiftcorresponding to a peak in the Raman spectrum of the material (see G. P.Agrawal, “Nonlinear Fiber Optics”, Academic Press Inc. (1995), pag.317-319). For example, for amplification of optical signals in a rangecomprised between 1460 and 1650 nm and using silica-based opticalfibers, the pump radiation wavelengths may be comprised approximatelybetween 1360 and 1550 nm, as the Raman shift of silica-based fibers isaround 100 nm in such wavelength range. Other materials may exhibit adifferent Raman shift. For the purposes of the present invention, it isconvenient to refer also to the optical frequency of the pump radiationemitted by the pump sources 13, 14, in addition to its wavelength.Furthermore, it is also convenient to express the Raman shift in termsof frequency. In fact, the Raman shift of a Raman-active material issubstantially constant in the range of frequencies of interest foroptical communications: for example, for silica-based orsilica/germania-based optical fibers the Raman shift is 13.2 THz. Thismeans that independently on the frequency of the pump radiation used, anamplification due to stimulated Raman scattering is obtained insilica-based or silica/germania-based optical fibers in a frequencyinterval around a maximum disposed at a frequency shifted of 13.2 THzwith respect to the pump frequency. Other materials (e.g.tellurite-based glasses) may present plural Raman peaks that can beexploited for Raman amplification. Advantageously, the frequency shiftcorresponding to the peak having maximum height may be chosen in orderto place the pump frequency range adapted for amplifying signals in agiven signal frequency range. Such choice may also be influenced byother factors, such as for example the width of the Raman peak, or theposition of the Raman peak. As a matter of fact, a thin peak may behardly used for designing a Raman amplifier, whereas the position of theRaman peak may be related, for example, to the availability of pumplasers having the desired frequency and power emission.

The Raman spectrum of a material may be obtained with conventionaltechniques. For example, it can be obtained by using the apparatus 70 asshown in FIG. 7, including a Raman-active fiber 71 under test, anunpolarized pump source 72 connected by a WDM coupler 73 to theRaman-active fiber 71, an optical spectrum analyzer 74 and an attenuator75. By coupling a pump radiation having whatever frequency and a powerof at least 200 mW in the Raman-active fiber 71 without any input signalto be amplified, it is possible to measure with the spectrum analyzer 74the spontaneous emission (ASE) caused in the Raman-active fiber 71 bythe pump radiation. The spectrum analyzer 74 is connected to the WDMcoupler 73 so as to measure the ASE propagating in opposite directionwith respect to the propagation direction of the pump. In such way, themeasure is not influenced by the high power of the pump radiationexiting from the Raman-active fiber 71 on the opposite side with respectto the input of the pump radiation. The attenuator 75 allows to avoidinjuries of an operator during the measure and/or to drastically reduceback-reflections of the pump radiation exiting at the end of theRaman-active fiber 71. Then, the Raman shift of the material included inthe Raman-active fiber 71 may be derived by the ASE spectrum as theabsolute value of the difference between the frequency of the peak ofthe ASE spectrum obtained at the spectrum analyzer 74 and the frequencyof the pump radiation used. The measure may be performed at roomtemperature. In case of a spectrum including different peaks, if themeasure is performed with the maximum pump frequency of the chosen pumpfrequency range, the Raman shift corresponds to the absolute value ofthe difference between the frequency of the highest peak included in afrequency range corresponding to the signal frequency range and thefrequency of the pump.

The obtaining of the Raman spectrum may be also performed with differentinput signals having respective different frequencies: by measuring thegain obtained by each signal frequency, the value of the Raman shift canbe obtained as the absolute value of the difference between the signalfrequency corresponding to the gain peak and the frequency of the pumpradiation used. The measure may be performed at room temperature. Incase of a spectrum including different peaks, if the measure isperformed with the maximum pump frequency of the chosen pump frequencyrange, the Raman shift corresponds to the absolute value of thedifference between the frequency of the highest peak included in afrequency range corresponding to the signal frequency range and thefrequency of the pump.

In order to obtain a significant amplification in a broad wavelengthrange of an optical signal, multiple pump frequencies are used in theamplifier 10 of the invention. The whole of the multiple pumpfrequencies provided by both pump sources 13, 14 extend over a widefrequency range, i.e. at least 40% of the Raman shift of the materialincluded in the Raman active fibers 11, 12, preferably at least 50% ofthe Raman shift of the material included in the Raman active fibers 11,12, more preferably at least 60% of the Raman shift of the materialincluded in the Raman active fibers 11, 12, even more preferably atleast 70% of the Raman shift of the material included in the Ramanactive fibers 11, 12.

The coupling devices 15 and 16 may be wavelength division multiplexers.In the preferred embodiment shown in FIG. 1, they introduce the pumpingradiation provided by the pump sources 13, 14 in an opposite directionwith respect to the optical signal to be amplified, so as to havecounter-propagating Raman amplification in the Raman-active opticalfibers 11, 12. A co-propagating configuration or a mixed co- andcounter-propagating configuration of the Raman amplifier 10 may also beused.

The first stage 10′ and the second stage 10″ of the Raman amplifier 10are configured so that they are substantially isolated with each otherin the pump wavelength range. In other words, the pump source 13 of thefirst stage 10′ substantially provides pump radiation only to the firstRaman-active fiber 11 and the pump source 14 of the second stage 10″substantially provides pump radiation only to the second Raman-activefiber 12. If any pump residual remains from the amplification process inany of the first or second stage 10′, 10″, this is substantially blockedor filtered, so that it cannot substantially propagate in the other ofthe amplifier stages 10′ or 10″. Pump residuals may also be caused byreflections induced by less-than-perfect coupling between the variouscomponents included in the amplifier 10, such as for example the WDMs15, 16 and the Raman-active fibers 11, 12. Practically, an absence ofresidual in the first Raman-active fiber length 11 of a pump radiationcoming from the second pump source 14 and not used for amplification inthe second Raman-active optical fiber 12 may correspond to a power ofsuch residual entering in the first Raman-active fiber 11 lower by atleast 10 dB, preferably lower by at least 13 dB, more preferably lowerby at least 20 dB, than the pump power coupled in the first Raman-activefiber 11 by the first pump source 13. In the same way, an absence ofresidual in the second Raman-active fiber length 12 of a pump radiationcoming from the first pump source 13 and not used for amplification inthe first Raman-active optical fiber 11 may correspond to a power ofsuch residual entering in the second Raman-active fiber 12 lower by atleast 10 dB, preferably lower by at least 13 dB, more preferably lowerby at least 20 dB, than the pump power coupled in the secondRaman-active fiber 12 by the second pump source 14.

With reference to the amplifier configuration shown in FIG. 1, thewavelength division multiplexers may provide the required isolationbetween the first stage 10′ and the second stage 10″ for the pumpwavelength range. For example, the wavelength division multiplexer 15,disposed between the first and the second Raman-active fiber 11, 12, maybe capable of extracting substantially all the pump radiation comingfrom the second pump source 14, not used for amplification in the secondlength of Raman-active fiber 12, i.e. the pump residual from the secondstage 10″, so that such pump residual substantially does not propagatein the first length of Raman-active fiber 11 in the first stage 10′.Such extracted pump residual may be used for monitoring purposes and/orgain control. Typically, an isolation level of at least 10 dB in thepump wavelength range may be sufficient for the wavelength divisionmultiplexers (at least for the WDM 15 in FIG. 1) for providing therequired isolation, as a great part of pump power coming from the pumpsources 13, 14 is used for Raman amplification in the Raman-activefibers 11, 12. Clearly, in the amplifier configuration shown in FIG. 1,substantially no pump radiation coming from the first pump source 13propagates in the second length of Raman-active fiber 12 of the secondstage 10″, except for possible pump reflections at the WDM 15, that canbe minimized by a proper coupling. In the counter-propagatingconfiguration shown in FIG. 1, if the WDM coupling device 15 cannotprovide by itself a sufficient level of isolation, a unidirectionaldevice may be interposed between the first stage 10′ and the secondstage 10″, so as to block substantially all the possible pump residualexiting from the second length of Raman-active fiber 12. Theunidirectional device may be an isolator or a circulator. The use of acirculator may further allow the elimination of the first couplingdevice 15, as the first pump source 13 may be suitably connected to oneof ports of the circulator. In other embodiments, for example inco-propagating configurations of the Raman amplifier 10, a wavelengthselective filter may be included between the first and the second stage10′, 10″, the wavelength selective filter being suitable for passing theoptical signal radiation and for filtering the pump radiation.

The pump source 13 included in the first stage 10′ provides a pumpradiation having at least one frequency included in a first group offrequencies. The first group of frequencies defines a first pumpfrequency range between a first minimum pump frequency and a firstmaximum pump frequency. The pump source 14 included in the second stage10″ provides a pump radiation having at least one frequency included ina second group of frequencies. The second group of frequencies defines asecond frequency range between a second minimum pump frequency and asecond maximum pump frequency. The first and the second group offrequencies are not coincident: more particularly, at least a portion ofthe frequencies included in the first group is not included in thesecond group and vice-versa. In a preferred embodiment, the first andthe second pump frequency ranges do not overlap with each other. Inanother embodiment, the frequencies of the first group may beinterleaved to the frequencies of the second group. The widths of eachof the first and the second pump frequency range do not exceed the 70%of the Raman shift of the material included in the Raman-active opticalfibers 11, 12, preferably the 60%, more preferably the 50%, even morepreferably the 40% of the Raman shift. In preferred embodiments of theRaman amplifier of the invention, each amplifier stage provides asubstantial amplification only of a portion of the traveling opticalsignal, while leaving other signal portions practically not amplified.In fact; with Raman amplification each pump frequency provides asubstantial amplification of an optical signal in a frequency rangehaving a width of only about 25-30% of the Raman shift of the materialincluded in the Raman-active fibers 11, 12, centered around a frequencyshifted of one Raman shift from the pump frequency. As Raman amplifiersaccording to the invention use different frequency sub-ranges for thepump radiation in different amplifier stages, each amplifier stageprovides substantial amplification only of the shifted frequencysub-range related to the pump frequency sub-range used in the stage,i.e. typically of a sub-range of the whole signal frequency range.

The allocation of the pump frequencies in the different amplifier stagesaccording to the indications given above allows to reduce, or possiblyto practically avoid, the transfer of energy between shorter pumpwavelengths and longer pump wavelengths in a Raman amplifier. Inparticular, in the multiple stage amplifier of the invention anytransfer of energy between shorter and longer pump wavelengths isreduced, or practically avoided, in all amplifier stages. The Applicanthas found that this may reduce the noise figure of the shorterwavelength signals in a broadband Raman amplifier. Let's consider, forexample, a first pump wavelength of 1425 nm (frequency ν_(p)=210.5 THz)and a second pump wavelength of 1505 nm (ν_(p)=199.3 THz). These firstand second pump wavelengths may be exploited for Raman amplification ofan optical signal comprised in a range between about 1520 and 1610 nm,in a Raman amplifier comprising silica-based or silica/germania-basedoptical fibers. The corresponding frequencies of these pump radiationsdiffer with each other of 11.2 THz, i.e., of about 85% of the Ramanshift of silica-based optical fibers. If such pump radiations werelaunched in the same Raman-active fiber, a significant portion of theoptical power of the first pump wavelength at 1425 nm would betransferred to the second pump wavelength at 1505 nm by stimulated Ramanscattering, so that such transferred portion would not be used foramplification of the optical signal. Consequently, the shorterwavelength portion of the optical signal would be less amplified withrespect to longer wavelength portion. Accordingly, the power of the pumpradiation at 1425 nm should be increased in order to take into accountof this effect. However, the transfer of energy between the pumpradiation at 1425 nm and the pump radiation at 1505 nm may cause theRaman amplifier to work in saturation regime at the shorter wavelengths.As a matter of fact, an amplifier works in saturation regime when anamplified radiation has almost the same power of the pump radiation in asignificant portion of the amplifying medium. In this case, the pumpradiation having longer wavelength, amplified by the pump radiationhaving shorter wavelength, may have almost the same power of the latterin a significant portion of the Raman-active fiber, even in case of anunbalanced launch power of the two pump radiations into the Raman-activefiber. Thus, the shorter pump wavelength may be deeply saturated by thelonger pump wavelength, with the inconvenient that the signal portionamplified by the shorter wavelength may have a higher noise figure. Infact, saturation is a more noisy regime for an amplifier with respect toa linear regime.

On the contrary, by configuring the Raman amplifier as a double stageRaman amplifier, by inserting the first pump wavelength of 1425 nm inthe first stage and the second pump wavelength of 1505 nm in the secondstage, and by isolating the first and the second stage in the pumpwavelength range, no transfer of energy may practically occur betweenthe pump wavelength at 1425 nm and the pump wavelength at 1505 nm, sothat each amplifier stage may work at a linear or at a low saturationregime, leading to a low noise figure in both stages, i.e., to anoverall low noise figure versus wavelength of the amplifier.

In case of use of more pump frequencies, for example in order to obtaina flat gain curve versus wavelength on a broad wavelength range, thedistance between the various frequencies has to be considered. Forexample, the several pump frequencies may be included in an overall pumpfrequency range having a width nearly equal to the Raman shift of thematerial exploited for Raman gain. Following the indications givenabove, the pumping radiations having a frequency which is distant lessthan the 70% of the Raman shift from the minimum pump frequency may befor example arranged in one amplifier stage and the pumping radiationshaving a frequency which is distant more than the 70% of the Raman shiftfrom the minimum pump frequency may be arranged in another amplifierstage. By providing a high isolation in the pump wavelength rangebetween the two amplifier stages, a transfer of energy between shorterand longer pump wavelengths in each amplifier stage is greatly reducedor even avoided, due to the relatively low distance between the minimumpump wavelength and the maximum pump wavelength in each amplifier stage.In particular, the lower the pump wavelength range in each amplifierstage, the lower the interactions between shorter and longer pumpwavelengths within each amplifier stage. Further, the isolation betweenthe first and the second amplifier stage may guarantee that practicallyno transfer of energy may occur between shorter and longer pumpwavelengths of the first and of the second amplifier stage. Accordingly,each amplifier stage may work at a linear or at a low saturation regime,leading to a low noise figure in both stages, i.e., to an overall lownoise figure versus wavelength of the amplifier.

Ideally, in case of use of several pump frequencies, a possible solutioncould be to arrange one isolated amplifier stage for each pumpfrequency, so as to completely avoid any transfer of energy betweendifferent pump frequencies. Each amplifier stage would provide, in thiscase, a substantial amplification only of a very small portion of thetraveling optical signal, leaving other signal portions practically notamplified. However, it has to be considered that the isolation requiredbetween the different stages in the pump wavelength range has to beprovided by suitable components (e.g. WDMs, isolators, wavelengthselective filters): each of these components introduces an insertionloss on the optical signal. A further source of loss on the travelingoptical signal is also represented by the Raman-active fibers includedin the different stages, at least for the portions of optical signalsubstantially not amplified in the first amplifier stages. In suchconditions, the last amplifier stages may receive a very poor level ofoptical signal power for some portions of optical signal not amplifiedin the first amplifier stages, with the consequence of a higher noisefigure for these portions of optical signal. Thus, when the number ofpump frequencies to be used is higher than or equal to three it may bepreferred to arrange at least two pump frequencies in a single stage.Preferably, at least one of the first and of the second pump frequencyranges has a width of at least the 20% of the Raman shift of thematerial included in the Raman-active fibers. This allows to reach agood compromise between the reduction of the transfer of energy betweendifferent pump wavelengths on one hand, with a sufficiently low numberof amplifier stages on the other hand, so as to maintain an acceptablelevel of power in the whole wavelength range of the optical signalwithin all the amplifier stages, and, consequently, an overall low noisefigure.

The different pump frequencies may be disposed in the first and in thesecond amplifier stage 10′, 10″ by preferably taking into account theattenuation versus wavelength of the Raman-active medium included in theRaman active fibers 11, 12. More particularly, in order to obtain a lownoise figure, a first portion of optical signal having a greaterattenuation in the Raman-active medium with respect to a second portionof optical signal should be preferably amplified first in the Ramanamplifier. Accordingly, the pump frequencies suitable for amplifyingsuch first portion of optical signal may preferably be arranged in thefirst amplifier stage, with respect to the traveling direction of theoptical signal to be amplified. For example, with reference to FIG. 1and considering a traveling direction of the optical signal to beamplified from left to right, the Raman amplifier 10 may comprisesilica-based fibers or silica/germania-based fibers and may be adaptedfor amplifying signals ranging from 1530 nm to 1610 nm. For thispurpose, the following four pump wavelengths may be used: 1425 nm(ν_(p)=210.5 THz), 1440 nm (ν₂=208.3 THz), 1470 nm (ν_(p)=204.1 THz),1510 nm (ν_(p)=198.7 THz). A preferred arrangement for these four pumpwavelengths may be disposing the first two wavelengths for pumping thefirst stage 10′ and the second two wavelengths for pumping the secondstage 10″. With such arrangement, the first amplifier stage 10′ providesa substantial amplification of a portion of optical signal having awavelength comprised in a range between about 1530 nm and 1560 nm, whilethe second amplifier stage 10″ provides a substantial amplification of aportion of optical signal having a wavelength comprised in a rangebetween about 1560 nm and 1610 nm. As the attenuation of an opticalsignal having a wavelength comprised between 1530 nm and 1560 nm ishigher with respect to the attenuation of an optical signal having awavelength comprised between 1560 nm and 1610 nm in a silica-based orsilica/germania-based optical fiber, it is convenient to dispose theshorter pumping wavelengths in the first amplifier stage 10′ and thelonger pump wavelengths in the second amplifier stage 10″. In fact, theportion of optical signal having higher wavelengths may travel throughthe first amplifier stage 10′ without suffering a too high attenuation,before being amplified in the second amplifier stage 10″, keeping thenoise figure of this portion of optical signal to an acceptable level.On the other hand, the portion of optical signal having lowerwavelengths is amplified in the first amplifier stage 10′, with nosubstantial degradation of its noise figure due to attenuation in thesecond amplifier stage 10″.

The arrangement of the pump frequencies according to the invention mayadvantageously lead to a Raman amplifier having a low noise figure and aflat, broad band Raman gain by using pump lasers having substantiallythe same pump power emission, i.e. having an overall variation of atmost the 50%, preferably the 40%, more preferably the 30% of the averagepump power emission of the pump lasers used in the Raman amplifier. Infact, as a transfer of energy is reduced or practically avoided betweenthe different pumping radiations, there is no need to use differentpowers for different pump wavelengths in order to reach a flat Ramangain versus wavelength. The use of pump lasers having substantially thesame pump power emission is of particular importance with reference todifferent aspects related to the design and to the functioning of theRaman amplifier, such as for example the reliability of the pump lasers(pump lasers having the same reliability can be used), the durability ofthe pump lasers (pump lasers will, on average, work fine for almost thesame time), the cooling of the pump lasers (the same type of coolingapparatus can be used for all the pump lasers), the manufacturingtechnology of the pump lasers (all the pump lasers may be of the samekind).

The Raman amplifier according to the invention may be part of a WDMtransmission system, comprising a transmitting station, a receivingstation and an optical line connecting said transmitting station andsaid receiving station. The transmitting station comprises a pluralityof transmitters adapted to emit a respective plurality of opticalchannels, each having a respective wavelength. The plurality of opticalchannels are combined together in a WDM optical signal by a multiplexingdevice, to be inserted into a first end of the optical line. Thereceiving station comprises a plurality of receivers adapted to receivethe WDM optical signal and discriminate the information carried by eachoptical channel received. For this purpose, the receiving stationtypically comprises a demultiplexing device, connected to a second endof the optical line, adapted to separate the different optical channelsincluded in the WDM optical signal. The optical line typically comprisesat least one transmission optical fiber. At least one amplifier,comprising at least one Raman amplifier according to the invention, isprovided along the optical line in order to counteract attenuationintroduced on the optical signal by at least a portion of saidtransmission optical fiber or fibers. Other sources of attenuation canbe connectors, couplers/splitters and various devices, such as forexample modulators, switches, add-drop multiplexers and so on, disposedalong the optical line, and/or in the transmitting station, and/or inthe receiving station. The optical transmission system comprising atleast one Raman amplifier according to the invention can be any kind ofoptical transmission system, such as for example a terrestrialtransmission system or a submarine transmission system. The optical linemay also comprise other types of amplifiers, such as for example erbiumdoped fiber amplifiers or semiconductor amplifiers, in combination withat least one Raman amplifier according to the invention. In particular,distributed Raman amplification may be used in combination withembodiments of lumped Raman amplifiers arranged according to theteachings of the present invention.

In particular, FIG. 2 shows a preferred embodiment of a lumped Ramanamplifier 10 according to the invention, comprising four stages 10 ^(i),10 ^(ii), 10 ^(iii), 10 ^(iv). Each of the four amplifier stagescomprises a Raman-active fiber (11, 12, 21, 22) and a WDM coupler (15,16, 25, 26). The four amplifier stages may also comprise opticalisolators (17, 18, 27, 28). A first pump source 13 is arranged so as toprovide pump radiation to the Raman-active fibers 11, 21 of the firstand of the third stage 10 ^(i), 10 ^(iii). For this purpose, a splittingdevice 19 may be used for dividing the radiation emitted by the pumpsource 13. A second pump source 14 is arranged so as to provide pumpradiation to the Raman-active fibers 12, 22 of the second and of thefourth stage 10 ^(ii), 10 ^(iv). For this purpose, a splitting device 20may be used for dividing the radiation emitted by the pump source 14.Any conventional splitting device 19, 20 may be used in the embodimentshown in FIG. 1, such as for example a fused fiber coupler or anintegrated waveguide coupler. With regards to the features of theRaman-active fibers 11, 12, 21, 22, of the WDM couplers 15, 16, 25, 26and of the pump sources 13, 14 reference is made to what disclosed abovein connection with FIG. 1. In particular, a first group of pumpfrequencies is used in the first and in the third amplifier stage and asecond group of pump frequencies is used in the second and in the fourthamplifier stage. The first and the second groups of pump frequencieshave the features disclosed above in connection with FIG. 1.

The preferred embodiment of Raman amplifier shown in FIG. 1 or FIG. 2may be advantageously used in metropolitan, long-haul or ultra long-hauloptical systems in place of erbium doped fiber amplifiers, foramplifying WDM or DWDM optical signals included between 1460 nm and 1650nm, typically between 1530 nm and 1610 nm. The amplifier of FIG. 1 orFIG. 2 may advantageously amplify an optical signal exploiting all theavailable wavelength band for the WDM optical signal. On the contrary,amplification based on erbium doped fiber amplifiers typically isperformed using amplifier schemes in which different stages disposed inparallel with each other between a demultiplexing and a multiplexingdevice are used for amplifying different wavelength bands. This has thedisadvantage that portions of bandwidth cannot be used for the opticalsignal, in order to correctly separate and route the differentwavelength bands in the different parallel amplifier stages.

Typically, in a long-haul or ultra-long-haul system a compensation ofthe chromatic dispersion is performed at the amplifier sites. Further,an optical add-drop multiplexer may be disposed in order to extract andinsert optical channels along the optical line, at the amplifier sites.A chromatic dispersion compensator and/or an optical add-dropmultiplexer may be advantageously disposed between the second and thethird amplifier stages in the preferred embodiment of Raman amplifier 10shown in FIG. 2.

EXAMPLE 1 Comparison

In a first simulation, the Applicant considered a double stage amplifieraccording to FIG. 3. Each of the first and second stage 30′, 30″ of theRaman amplifier included a length of 3500 m of a silica-germaniaRaman-active optical fiber (31, 32) having a g_(R)/A_(eff) ratio of6.5.10⁻³ ¹/(W·m), an attenuation of 0.4 dB/km at a signal wavelengthbetween 1530 nm and 1610 nm and an attenuation of 0.6 dB/km at a pumpwavelength range included between 1425 nm and 1510 nm. Four differentpump wavelengths were used for pumping both Raman-active fibers 31, 32:λ₁=1425 nm (ν₁=210.5 THz), λ₂=1440 nm (ν₂=208.3 THz), λ₃=1470 nm(λ₃=204.1 THz), λ₄=1510 nm (λ₄=198.7 THz). More particularly, in thepump source 33 the pump power emission of four pairs of pump lasers (onepair per pump wavelength, each pair consisting of two lasers connectedto a polarization beam combiner) were multiplexed together and sent, viathe 60/40 splitter 39 and the WDM couplers 35, 36 to both Raman-activefibers 31, 32. More particularly, the 60% of the pump radiation was sentto the first stage and the 40% to the second stage. In each amplifierstage, the difference between the minimum and the maximum pump frequencywas 11.8 THz (i.e., about 89% of the Raman shift).

The noise figure of a Raman amplifier arranged as above was calculatedfor two different configurations, being optimized for obtaining:

-   -   a) a gain of 10 dB, with an input signal power of −11 dBm per        channel, (suitable for an ultra long-haul system using 200        channels with a channel spacing of 50 GHz);    -   b) a gain of 28 dB, with an input signal power of −28 dBm per        channel, (suitable for a long-haul system using 200 channels        with a channel spacing of 50 GHz).

In the following table 1 the pump power P(λ) per each pump laser isreported for the two configurations a) and b) optimized for fulfillingthe above performances.

TABLE 1 Configuration P (λ₁) P (λ₂) P (λ₃) P (λ₄) a) 560 mW 450 mW 160mW 70 mW b) 760 mW 620 mW 180 mW 30 mW

A medium access loss of 6 dB between the first stage 30′ and the secondstage 30″ was also considered, for example in case of accomplishment ofa dispersion compensator. The obtained gain flatness between 1530 nm and1610 nm was of ±0.5 dB for both configurations a) and b).

FIG. 4 shows the obtained noise figure versus wavelength between 1530and 1610 nm for configurations a) (curve 41) and b) (curve 42). As itcan be seen, a higher noise figure is obtained for shorter signalwavelengths with respect to longer signal wavelength in both cases, dueto transfer of energy between shorter pump wavelengths and longer pumpwavelengths. Due to this energy transfer, the pump radiation at shorterpump wavelengths must have high power, which is not used foramplification of the signal.

EXAMPLE 2 Invention

In a second simulation, the Applicant considered a configuration ofRaman amplifier according to FIG. 2. The same pump wavelengths disclosedwith reference to example 1 were used, but this time the pump source 13included only pump lasers having wavelengths λ₁, λ₂ and the pump source14 included only pump lasers having wavelengths λ₃, λ₄ (one pair oflasers per wavelengths, combined together via a polarization beamcombiner). The features of the Raman-active fibers used in the fourstages of the amplifier were also the same as in example 1. The splitter19 was a 60/40 splitter, the splitter 20 was a 65/35 splitter. Moreparticularly, the higher pump power was sent to the first and thirdamplifier stages. Thus, in the first and in the third amplifier stagesthe difference between the minimum and the maximum pump frequency was 1THz (i.e. about 7.5% of the Raman shift), whereas in the second and inthe fourth amplifier stages the difference between the minimum and themaximum pump frequency was 5.4 THz (i.e. about 41% of the Raman shift).

For fulfilling the conditions required in case a) of example 1 (ultralong-haul system) the first Raman-active fiber 11 was 1500 m long, thesecond Raman-active fiber 12 was 1400 m long, the third Raman-activefiber 21 was 1800 m long and the fourth Raman-active fiber 22 was 2000 mlong. All the pump lasers had a power emission of 400 mW.

For fulfilling the conditions required in case b) of example 1(long-haul system) the first Raman-active fiber 11 was 2000 m long, thesecond Raman-active fiber 12 was 2000 m long, the third Raman-activefiber 21 was 3000 m long and the fourth Raman-active fiber 22 was 3000 mlong. All the pump lasers had a power emission of 415 mW.

A medium access loss of 6 dB was also considered in both cases, betweenthe second and third stage, for example in case of accomplishment of adispersion compensator. The obtained gain flatness between 1530 nm and1610 nm was of ±0.5 dB in both cases.

FIG. 5 shows the obtained noise figure versus wavelength between 1530and 1610 nm for configurations a) (curve 51) and b) (curve 52). As itcan be seen, a lower noise figure is obtained at shorter signalwavelengths with respect to the previous cases shown in FIG. 4, due to asubstantial reduction of transfer of energy between shorter and longerpump wavelengths within each amplifier stage. More particularly, curve51 in FIG. 5 is lower than the corresponding curve 41 in FIG. 4 over thewhole wavelength range; on the other hand, curve 52 in FIG. 5 is lowerthan the corresponding curve 42 in FIG. 4 up to about 1590 nm. Forlonger signal wavelengths, a slightly higher noise figure is obtainedwith respect to the previous example in case b), due to the fact thatthe longer signal wavelengths are practically amplified only in thesecond and fourth stage of the amplifier, so that they suffer a higherinput loss in comparison with the shorter signal wavelengths. However,for configuration b) a maximum value of about 5.9 dB is obtained, withrespect to about 7 dB in FIG. 4.

EXAMPLE 3 Invention

In a third simulation, the Applicant considered a further configurationof Raman amplifier according to FIG. 2. The same pump wavelengthsdisclosed with reference to example 1 were used, but this time the pumpsource 13 included pump lasers having wavelengths λ₁, λ₂, λ₃ and thepump source 14 included only pump lasers having wavelengths λ₄ (one pairof lasers per wavelengths, combined together via a polarization beamcombiner). The features of the Raman-active fibers used in the fourstages of the amplifier were also the same as in example 1. The splitter19 was a 60/40 splitter, the splitter 20 was a 65/35 splitter. Moreparticularly, the higher pump power was sent to the first and thirdamplifier stages. Thus, in the first and in the third amplifier stagesthe difference between the minimum and the maximum pump frequency was6.4 THz (i.e., about 49% of the Raman shift). For fulfilling theconditions required in case a) of example 1 (ultra long-haul system) thefirst Raman-active fiber 11 was 1800 m long, the second Raman-activefiber 12 was 2000 m long, the third Raman-active fiber 21 was 2000 mlong and the fourth Raman-active fiber 22 was 2200 m long. Forfulfilling the conditions required in case b) of example 1 (long-haulsystem) the first Raman-active fiber 11 was 2000 m long, the secondRaman-active fiber 12 was 3000 m long, the third Raman-active fiber 21was 2500 m long and the fourth Raman-active fiber 22 was 2500 m long.

In the following table 2 the pump power P(λ) per each pump laser isreported for the two configurations a) and b) optimized for fulfillingthe required performances.

TABLE 2 Configuration P (λ₁) P (λ₂) P (λ₃) P (λ₄) a) 450 mW 450 mW 200mW 400 mW b) 600 mW 550 mW 150 mW 500 mW

A medium access loss of 6 dB was also considered in both cases, betweenthe second and third stage, for example in case of accomplishment of adispersion compensator. The obtained gain flatness between 1530 nm and1610 nm was of ±0.5 dB in both cases.

FIG. 6 shows the obtained noise figure versus wavelength between 1530and 1610 nm for configurations a) (curve 61) and b) (curve 62). As itcan be seen, a substantially flat noise figure versus wavelength isobtained. More particularly, for case a) a maximum noise figure value ofabout 7.4 dB is obtained, with respect to about 8.7 dB in FIG. 4,whereas for configuration b) a maximum noise figure value of about 6.2dB is obtained, with respect to about 7 dB in FIG. 4.

1-19. (canceled)
 20. A Raman amplifier adapted for amplifying an opticalsignal having frequency in a signal frequency range comprising at leasta first and a second optical path disposed in series with each other,said amplifier comprising: a first pump source connected to said firstoptical path for emitting and coupling into said first optical path afirst pump radiation including a first group of pump frequencies; and asecond pump source connected to said second optical path for emittingand coupling into said second optical path a second pump radiationincluding a second group of pump frequencies; wherein at least a portionof said first group of pump frequencies is excluded from said secondgroup of pump frequencies and at least a portion of said second group ofpump frequencies is excluded from said first group of pump frequencies.